React.js and PHP together?
I've used PHP and React together briefly (and then moved on to isomorphic React). Running V8 with PHP is only useful if you need server-side rendering (for fast initial loading and easy SEO), but I've not tried it so I'm not sure whether it's stable/reliable enough. If you do not care about server-side rendering, then you can just build the React app and include it in your PHP view.
Basically, the PHP view would serve as a layout, with a React container element defined so that your React app can bootstrap with it.
You can also pre-fetch the initial data for the React app with PHP and somehow attach it to your PHP view. The simplest way would be to use a script block to assign the JSON-serialized data to a global variable. Another way would be to define element(s) and attach your JSON-serialized data as element attributes, to avoid globals. Either way, you'd have your Flux stores bootstrap with those initial data to avoid having to hit APIs before the app can load.